<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>外边距塌陷</title>
    <style>
        .father {
            width: 300px;
            height: 300px;
            background-color: pink;
            border-top: 1px solid #000;
        }

        .son{
            width: 100px;
            height: 100px;
            background-color: orange;
            margin-top: 50px;
        }
    </style>
</head>

<body>
    <div class="father">
        <div class="son">son</div>
    </div>
</body>

</html>